1. Field of the Invention
The present invention relates to a functional particle and a method of manufacturing the same.
2. Description of the Related Art
A toner used for elecrophotographic image formation contains a binder resin, a colorant, a release agent and the like. A typical method of manufacturing the toner includes a pulverization method. According to the pulverization method, a toner of an infinite form is manufactured by cooling to solidify a molten kneaded product of a binder resin, a colorant, a wax and the like and mechanically pulverizing the obtained solidification product. In the toner, since a fractured surface during pulverization appears on the surface, the colorant is often exposed to the surface. Since the colorant exposed to the surface gives an effect on the charging performance of the toner, this varies the charging performance of the toner. As a result, image defects such as unevenness in images tends to occur and images at high quality cannot be formed. It is extremely difficult to control the surface state of the toner in the reduction of the particle size by pulverization. For making the charging performance of the toner uniform, it is important that the colorant is not exposed to the toner surface. Further, it is important that the shape of the toner is uniform and the width of the grain size distribution is narrow.
Further, the release agent contained in the toner has a property of bleeding out to the toner surface with time. Since the release agent has tackiness, it tends to cause aggregation (blocking) between the toners. In a case of using a two-component developer containing a toner and a carrier, a phenomenon referred to as filming in which the release agent in deposited to the carrier surface occurs, which deteriorates the carrier and makes charging of the toner insufficient. On the other hand, the amount of the release agent in the toner is decreased by the bleed-out of the release agent. Accordingly, this tends to frequently generate an offset phenomenon that the toner is deposited not to the recording medium but to a fixing roller as a member for fusing toner images to the recording medium, to lower the fixing property of the toner to the recording medium. For eliminating the blocking, filming and offset phenomena, it is important to prevent bleed-out of the release agent to the toner surface. Further, for reducing the power consumption, a toner containing a binder resin having a relatively lower glass transition temperature and with low fixing temperature has been developed. However, since the binder resin tends to be softened by heat, the toner tends to cause blocking. In a case of using this toner, since the range for the possible fixing temperature is narrowed, it requires to conduct temperature control accurately during fixing which renders the control complicated during fixing. In order to eliminate blocking, it is important to prevent toner from being in contact with each other in a state where the binder resin is softened.
In view of the foregoing problems, an encapsulated toner in which a coating layer is formed on the toner surface has attracted attention. In a case of forming the coating layer on the toner surface, it is possible to conceal the colorant exposed to the toner surface, reduce the bleed-out of the release agent and, further, prevent contact between the toners in the softened state. Accordingly, various proposals have made for the encapsulated toner. For example, an encapsulated toner obtained by spraying a methylethyl ketone solution of polybutadiene to the periphery of a core material by a spray drying method and removing a solvent in a high temperature air is proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 4-174861 (1992). However, the spray drying method inevitably forms coarse coagulates and increases the width of the grain size distribution to vary the charging performance of the toner. Further, by the manufacturing method of JP-A 4-174861, a great amount of vapors of methylethyl ketone as an organic solvent is formed, which cannot be exhausted as it is in atmospheric air. Therefore, it needs a special recovery facility and is not suitable to production in an industrial scale.
Further, an encapsulated toner containing a colored resin particle as a granulation product of a binder resin containing a colorant (core particle), a release agent layer formed to the surface of the colorant resin particle and a resin coating layer formed on the surface of the release agent layer and comprising resin particle for encapsulation (shell particle) has been proposed (for example, in Japanese Unexamined Patent Publication JP-A 2001-324831). According to the technique of JP-A 2001-324831, a precursor particles for core particle in which a colorant and a release agent not compatible with the binder resin are dispersed in the binder resin is at first prepared by a pulverization method. A resin particle for encapsulation is deposited on the surface of the precursor particles by a mechanical impact force or dry mechanochemical method. Then, the precursor particle deposited with the resin particle for encapsulation is exposed to a hot air stream to fuse the resin particle for encapsulation to the precursor particle to form a resin coating layer. At the same time, the release agent is leached from the precursor particle to make the precursor particles into a colored resin particle, and a release agent layer is formed between the colored resin particle and the resin coating layer to prepare an encapsulated toner of JP-A 2001-324831. However, since the mechanical impact force or dry mechanochemical method has to be applied in an air stream at low particle concentration and the production efficiency is low, it is not suitable to the production in an industrial scale. Further, the resin coating surface is not sometimes formed over the entire surface of the colored resin particle to possibly vary the charging performance by the surface exposure of the colorant, etc.
On the other hand, a wet method of manufacturing a toner by utilizing an aggregating effect of particles in an aqueous medium has also been well known. The advantage of the wet method is that the shape of the obtained toner is uniform, and the width of the grain size distribution is relatively narrowed. That is, problems in the toner may possibly be overcome all at once by preparing the encapsulated toner by the wet method. For example, it has been proposed a manufacturing method of mixing a toner raw material mixture containing a resin exhibiting dispersibility to water by a neutralizing agent (hereinafter referred to as “self-dispersible resin), a colorant, a fine wax particle, and an organic solvent, and an aqueous medium under the presence of a neutralizing agent and conducting phase-inversion emulsification (for example, refer to Japanese Unexamined Patent Application JP-A 10-186714 (1998)). According to the manufacturing method of JP-A 10-186714, an encapsulated toner as a self-dispersible resin particle incorporating the colorant and a wax fine particle is obtained. The manufacturing method involves a problem that aggregation of the colorant tends to occur upon mixing the toner raw material mixture and the aqueous medium due to the less dispersibility of the colorant to water. The coagulant of the colorant induces aggregation of resin particles. Further, aggregation of the colorant varies the colorant content in the finally obtained encapsulated toner to make the charging performance not uniform.
Further, it has been proposed a method of manufacturing an encapsulated particle by batchwise treating a primary particle (core particle) and a secondary particle (shell particle) by a homogenizer and aggregating the secondary particles to the surface of the primary particle (for example, Japanese Unexamined Patent Publication JP-A 63-278547 (1988)). The number average particle size of the primary particle (core particle) is from 0.1 to 100 μm. The number average particle size of the secondary particle is ⅕ or less of the number average particle size of the primary particle. The spray pressure in the homogenizer treatment is 29.4 MPa (300 kgf/cm2) or more. In the technique of JP-A 63-278547, it is necessary to pressurize at 54.8 MPa or higher in order to prevent the occurrence of excess aggregation and obtain a particle of uniform grain size. The homogenizer used in the technique of JP-A 63-278547 is a homogenizer, for example, of a type of colliding a dispersion product at a high pressure against each other (for example, microfluidizer) or a homogenizer of a type of colliding a dispersion product at a high pressure against the inner wall (for example, Manton Gaulin homogenizer) according to JP-A 63-278547, p3, column 5, lines 8 to 18. Since each of the homogenizers has no coiled pipeline, less centrifugal force is added even when the shearing force is added. Accordingly, aggregation occurs between the primary particles to each other or between secondary particles to each other and the aimed encapsulated particles cannot be obtained by a yield at an industrially satisfactory level. In addition, the grain size of the obtained encapsulated particles is not uniform and the width of the grain size distribution is broad. Further, in the technique of JP-A 63-278547, since aggregation is conducted at a high pressure of 29.4 MPa or higher and at 54.8 MPa or higher depending on the case, a pressure proof facility and an arresting facility are indispensable for the practice of an industrial scale and increase in the size of the homogenizer is also required, so that this is not a practical method. Further, since only the secondary particles having a volumic average grain size ⅕ or less of the volumic average particle size of the primary particle can be used, usable secondary particles are restricted.